In recent years, printed electronics using a printing technique has been attracting attention in the field of production of electronic devices. In particular, there is increasing emphasis on conductive inks which can be formed into conductive lines by a coating process. Processes using conductive inks allow a considerably greater reduction in cost and environmental load than conventional vacuum processes, and thus have been ranked as industrially very important techniques. The development of conductive inks and materials usable therein has been actively pursued.
Conventional conductive inks are typically a metal paste obtained by mixing metal particles having a size on the order of micrometers with a binder resin and a solvent. Such metal pastes are widely used in electronics products such as printed boards. The metal pastes, however, have to be sintered at 200 to 300° C. to exhibit electrical conductivity. It has thus been a challenge to achieve high electrical conductivity through sintering at a lower temperature.
Under these circumstances, there have recently been developed low-temperature-sinterable silver nanoparticle inks that are capable of exhibiting a high electrical conductivity of 10−5 Ω·cm or less when sintered at a temperature of 150° C. or lower. For example, Patent Literature 1 discloses a method for producing a silver nanoparticle ink that exhibits an electrical conductivity of 10−5 Ω·cm even when heated at a temperature of 100° C. or lower. The silver nanoparticles have a structure in which silver in the form of nanoparticles is covered with protective organic molecules acting as a surfactant. The action of the protective organic molecules allows the silver nanoparticles to be dispersed relatively stably in various organic solvents. Some of the protective organic molecules are desorbed from the silver nanoparticles even at room temperature, by virtue of which the silver nanoparticles exhibit high electrical conductivity even when sintered at a low temperature. Silver nanoparticles having such characteristics have made it possible to use a low-temperature process to form an electronic circuit on a plastic film having low heat resistance.
Also, the development of techniques for producing an electronic circuit or a semiconductor device by a printing process using such an conductive ink has been in progress. Conventional metal pastes have high viscosities ranging from several thousands of cps to several hundreds of thousands of cps and are often used, for example, in screen printing or gravure printing with which such metal pastes are highly compatible in terms of printability. However, the width of conductive lines printable by these printing techniques is too large. Thus, there is an increasing demand for a silver nanoparticle ink for printing devices that allows the formation of finer conductive lines.
Silver nanoparticle inks have low viscosities ranging from several to a dozen or so cps and are thus considered to be well compatible with inkjet devices. However, the use of a silver nanoparticle ink in an inkjet device for forming fine conductive lines may suffer from a problem concerning the dispersion stability of silver nanoparticles. Non-uniform dispersion of silver nanoparticles in an ink may lead to clogging with the ink in an inkjet head or to a failure of the head to discharge ink droplets straight. This precludes precise drawing of conductive lines. In inkjet devices, the amount of ink droplets discharged from the head is tiny (on the order of picoliters), and the above problem concerning the dispersibility of silver nanoparticles is likely to emerge. Silver nanoparticle inks are therefore required to be improved particularly in terms of dispersibility.
In general, it is desirable for a solvent of an ink used in a printing device such as an inkjet device to have a high boiling point of 150° C. or higher in order to prevent a defect such as clogging caused by drying of the ink. In forming finer conductive lines, the ink more readily dries due to the smaller size of ink droplets discharged. Thus, in some cases, a solvent having a higher boiling point (e.g., 200° C. or higher) is used. For example, Patent Literature 2 discloses a silver nanoparticle dispersion in which dihydroxy terpineol having a boiling point of 200° C. or higher is used as a solvent.